Immobile Fluids
This node should be used to model an unsaturated porous matrix for which the pore space is not filled with liquid and contains one or more gas considered as immobile fluids; or a porous matrix containing insulated enclosures.
It is possible to add and define up to five different immobile fluids and volume fractions to the porous matrix.
Model Input
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups are added, the model inputs appear here.
Temperature
This section is available when temperature-dependent material properties are used. By default the temperature of the parent interface is used and the section is not editable. To edit the Temperature field, click Make All Model Inputs Editable (). The available options are User defined (default), Common model input (the minput.T variable, set to 293.15 K by default) and all temperature variables from the physics interfaces included in the model. To edit the minput.T variable, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Immobile Fluids
In this section, the Number of fluids to define — 1, 2, 3, 4, or 5 — is set. Depending on the Number of fluids, further settings display underneath. For each fluid:
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The material must be set from the Fluid material list, which can point to any material in the model.
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The Volume fraction of the immobile fluid θg {1,2,3,4,5} should be set.
The total volume fraction of immobile fluids is calculated from
The volume fraction available for mobile fluids (that is, the effective porosity) is then calculated from
where the total volume fraction of immobile solids is calculated from
Heat Conduction
For the same number of fluids selected under Immobile Fluids, the defaults for the Thermal conductivity kg use values From material. For User defined select Isotropic, Diagonal, Symmetric, or Full based on the characteristics of the thermal conductivity and other values or expressions in the fields or matrices.
The effective conductivity for the equivalent immobile fluid is calculated from
When one or more than one solid is selected in the Immobile Solids section, the effective conductivity of immobile solids and immobile fluids can be calculated in three different ways:
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If Volume average is selected under Effective Thermal Conductivity, the effective conductivity of the solid-fluid system is given by
where kp and kg are the effective conductivities of immobile solids and fluids.
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If Reciprocal average is selected under Effective Thermal Conductivity, the effective conductivity is calculated from
where kp and kg are the effective conductivities of immobile solids and fluids.
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If Power law is selected under Effective Thermal Conductivity, the effective conductivity is calculated from
where kp and kg are the effective conductivities of immobile solids and fluids.
Thermodynamics
For the same number of fluids selected under Immobile Fluids, the following properties should be set:
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Density ρg{1,2,3,4,5}
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Specific heat capacity Cp,g{1,2,3,4,5}
The effective volumetric heat capacity of the composite solid-fluid system is defined as
The Immobile Fluids node requires the Subsurface Flow Module. For a detailed overview of the functionality available in each product, visit https://www.comsol.com/products/specifications/
Location in User Interface
Context Menus
If the Porous matrix model is set to Extended under the Physical Model section:
Heat Transfer in Porous Media>Porous Medium>Immobile Fluids
More locations are available if the Heat transfer in porous media check box is selected and Porous matrix model is set to Extended under the Physical Model section. For example:
Heat Transfer in Solids>Porous Medium>Immobile Fluids
Ribbon
Physics Tab with Porous Medium selected in the model tree:
Attributes>Porous Medium>Immobile Fluids